C. gunnari have large heads, with depressed, elongate snouts. Their mouth is large with small teeth, all designed to capture their prey of krill and other plankton forms. They are fusiform in shape, tapered at the head and tail, with rounded pelvic fins (Kock 2005a). in C. gunnari and species of the genus Chionodraco, sexually mature males have a significantly higherfirst dorsal fin than females (Iwami and Kock 1990). Due in part to their lack of hemoglobin, mackerel icefish are predominantly blue and silver hues, with clear, almost white gills.

The oxygen carrying capacity of icefish blood is only 10% of what it would be for a fish with hemoglobin (Wojcik 2007). Blood is transparent and less viscous. C. gunnari has several adaptations to compensate for lacking hemoglobin. Its heart is significantly larger than its close relatives who have retained hemoglobin. This enables mackerel icefish to circulate blood volumes of blood 2-4 times greater than fish with hemoglobin (Wojcik 2007). The cold water temperature and thus relatively high concentration of oxygen and relatively low metabolic rates of C. gunnari also help compensating for lack of hemoglobin.

Because mackerel icefish have increased vascularitzation (higher number and density of veins and arteries) of their fins and scaleless skin it has been suggested that they utilize cutaneous respiration (Feller and Gerday 1997), obtaining oxygen through their skin, though this hasn’t been demonstrated through observation or experimentation.